[p_l]

VT5xx was the budget line with limited functionality, that's why only 525 among them supports ANSI color. The only fancy stuff was multisession (TD/SMP if you have all bits to support it) and "desktop accessories" like clock and calculator.

The really interesting ones are VT340 variants with ReGIS and full SIXEL graphics

[dgl]

The VT3xx ones that were color did not support ANSI SGR to set them. I don't think VT5xx was a budget line, it has more escape sequences than the previous ones (including interesting ones like changing cursor shape, which modern terminals implement too). It's more that they never made a graphical version of the VT5xx (this was the early 90s, whether physical VTs made sense anymore is debatable, but terminal graphics likely didn't).

VT340 is definitely interesting and if someone were to emulate one that would also be great! (there's been some good research, e.g.: https://github.com/hackerb9/vt340test, which you might be surprised to learn has been used to make Windows Terminal one of the more conformant terminals...)

[p_l]

The references to "budget line" were from contemporary sources, and that it was heavily reduced in cost - while terminal sequences were more of "simple matter of programming" ;-) My understanding that some of the features like changing cursor shape were related to the rudimentary custom font feature which was retained from more expensive 3xx line and which accepted IIRC sixel-encoded bitmap fonts.

Funnily enough, the one VT510 I had for some time actually came from a VHS rental place that for reasons unknown to me ran Blockbuster Video customized VAX 6.1 on Alpha (which I also grabbed). BBV was not very well known in Poland, but this specific machine had unwiped disks and logs showing it was used from 1996 to 2000, then it was found lying in a corner when a moving service was asked to clean out a location after a tenant that came in after the video rental.

There was probably a DECserver missing somewhere in the pile before I got my hands on it ;)